光衍射数值模拟中不同抽样方法的适用性分析

基于标量衍射理论和快速傅里叶变换,分析了光衍射的数值模拟时可以采用的不同抽样方法及其适用性。理论分析表明,按照抽样对象的不同,存在三种适用范围不同的算法:传递函数抽样法、点扩展函数抽样法和加权函数抽样法。其适用范围分别为衍射距离小于、大于和等于特征距离的情况。在Matlab软件环境下给出了三种算法的模拟计算实例,并与相应的实验结果进行了比较,证明了理论分析的正确性。最后还给出了一种无需改变抽样点数和抽样间隔就可计算任意距离衍射过程的自适应抽样算法及其在数字全息再现中的应用实例。     

蒙特卡罗粒子输运软件JMCT抽样工具库设计

为通用型蒙特卡罗粒子输运程序JMCT设计了抽样工具库,通过两种技术途径提供各分布的抽样。一是针对各种常见分布提供特定抽样子程序;二是提供一个通用型的抽样子程序,可以实现任意离散分布和任意一维有限区间上连续分布的自动抽样。在设计任意一维有限区间上连续分布的自动抽样工具时利用了部分开源代码,利用其功能提供给用户最大的方便性。对抽样工具库的检验表明,其可以正确、方便地实现各种输运模拟中常见分布的抽样。     

Defining the sampling space in multidimensional NMR experiments: What should the maximum sampling time be?

Efficient sampling of signals is a key issue for multiple-dimensional NMR experiments to establish the best ratio between experiment time and spectral quality. Focussing on the most widely used sampling strategy using standard rectangular sampling and data analysis by Fourier transformation, a central question is concerned with determining the optimal maximum sampling time in the individual dimensions. The spectral resolution depends directly on this choice, as do the overall experiment times when addressing the indirect dimensions. We present a theoretical, numerical, and experimental analysis of the sampling space problem and propose approaches to efficient sampling for typical cases.     

Randomization improves sparse sampling in multidimensional NMR

While a number of strategies have been developed to reduce data collection requirements for multidimensional NMR based on non-Fourier methods of spectrum analysis, there is an increasing awareness that the principal differences in the performance of these methods is attributable to the sampling strategies employed, and not the method of spectrum analysis per se. The ability of maximum entropy reconstruction to utilize essentially arbitrary sampling schemes makes it a useful platform for comparative analysis of sampling strategies. Here we use maximum entropy reconstruction to demonstrate that artifacts characteristic of sparse sampling result from regularity in the sampling pattern, and that they can be substantially reduced by introducing a degree of randomness to an otherwise regular sampling scheme, without requiring additional sampling.     

Optimal polar image sampling

In this paper, a problem of efficient image sampling (deployment of image sensors) is considered. This problem is solved using techniques of two-dimensional quantization in polar coordinates, taking into account human visual system (HVS) and eye sensitivity function. The optimal radial compression function for polar quantization is derived. Optimization of the number of the phase levels for each amplitude level is done. Using optimal radial compression function and optimal number of phase levels for each amplitude level, optimal polar quantization is defined. Using deployment of quantization cells for the optimal polar quantization, deployment of image sensors is done, and therefore optimal polar image sampling is obtained. It is shown that our solution (the optimal polar sampling) has many advantages compared to presently used solutions, based on the log-polar sampling. The optimal polar sampling gives higher SNR (signal-to-noise ratio), compared to the log-polar sampling, for the same number of sensors. Also, the optimal polar sampling needs smaller number of sensors, to achieve the same SNR, compared to the log-polar sampling. Furthermore, with the optimal polar sampling, points in the image middle can be sampled, which is not valid for the log-polar sampling. This is very important since human eye is the most sensitive to these points, and therefore the optimal polar sampling gives better subjective quality.     

不同采样方式下光纤布喇格光栅反射谱寻峰算法的分析

对光纤布喇格光栅反射谱的三种寻峰算法(三次样条插值数值微分法、高斯-多项式拟合法和高斯拟合法)进行了分析和比较;相同采样情况下,得出了高斯拟合法确定的峰值准确度最高的结论;采样点数为250的均匀采样中,三种算法寻峰结果对实际值的误差分别为:3.4pm、13.0 pm和2.6 pm.引入了非均匀数据采集的寻峰思路,分别应用三种寻峰法对实际光栅的反射谱分别进行均匀采集和非均匀采集.结果表明,对于相同寻峰方法在非均匀数据采集下获得的峰值更精确,采样点数为250时,高斯拟合法寻峰时非均匀采集对应的误差比均匀采集减少了38.46％.     

不同采样方式下光纤布喇格光栅反射谱寻峰算法的分析

对光纤布喇格光栅反射谱的三种寻峰算法(三次样条插值数值微分法、高斯-多项式拟合法和高斯拟合法)进行了分析和比较;相同采样情况下,得出了高斯拟合法确定的峰值准确度最高的结论;采样点数为250的均匀采样中,三种算法寻峰结果对实际值的误差分别为: 3.4 pm、13.0 pm和2.6 pm.引入了非均匀数据采集的寻峰思路,分别应用三种寻峰法对实际光栅的反射谱分别进行均匀采集和非均匀采集.结果表明,对于相同寻峰方法在非均匀数据采集下获得的峰值更精确,采样点数为250时,高斯拟合法寻峰时非均匀采集对应的误差比均匀采集减少了38.46%.     

Signal reconstruction from the undersampled signal samples

It is well-known from the celebrated Shannon sampling theorem for bandlimited signals that if the sampling rate is below the Nyquist rate, aliasing takes place and the original signal cannot be reconstructed back by simply passing the signal samples through an ideal lowpass filter. However, researchers such as Stern and Gori have shown the existence of some classes of signals for which the signals are sampled below the Nyquist rate but perfect signal reconstruction is still possible from the given signal samples. Here, we present a generalized lowpass sampling theorem and show that Stern’s and Gori’s lowpass sampling theorems are special cases of it. A sampling theorem for the bandpass signals in the linear canonical transform domains is also presented and its special cases are discussed. Using a modification of the conventional natural sampling waveform with a specific width of the pulses, it is shown that the sampling rate in our generalized lowpass sampling theorem and hence in the Stern’s and the classical Shannon sampling theorems can be further reduced by a factor of two, while for the bandpass signals, the reduction in the sampling rate by some factor is possible only under some restricted conditions.     

基于高速肖特基二极管的100 ps瞬态取样门设计与仿真

 皮秒级瞬态取样门主要应用于激光聚变实验和高能物理实验中，对单次高速脉冲进行实时取样。提出了一种新颖的基于肖特基二极管桥的平衡取样门，给出其模型和具体电路设计。电路仿真结果表明，对称的选通设计保证了选通脉宽为100 ps时，取样间隔也为100 ps，取样门带宽为4.4 GHz，可应用于多路超短激光脉冲取样。     

根据稀疏基底选择抽样模型

在压缩传感技术应用中,根据稀疏基底选择抽样模型对重构结果影响很大。在傅里叶空间中,极坐标星形抽样和随机抽样的重构效果差异巨大,应用傅里叶光学理论对傅里叶空间的频谱分布进行分析,从理论上解释了原因,并且据此提出稀疏基底和抽样模型的匹配情况会影响重构效果。在小波空间中,进行了均匀抽样和随机抽样的对比重构实验,发现后者的重构效果更好,并确定了根据稀疏基底选择合适抽样模型的可行性,为实际应用中降低抽样率,提高重构效果提供了方法依据。     

Multichannel sampling theorem for bandpass signals in the linear canonical transform domain

The linear canonical transform (LCT) describes the effect of first-order quadratic phase optical system on a wave field. The classical multichannel sampling theorem for common bandlimited signals has been extended differently to bandlimited signals associated with LCT. However, a practical issue associated with the reconstruction of the original bandpass signal from multichannel samples in LCT domain still remains unresolved. The purpose of this paper is to introduce a practical multichannel sampling theorem for bandpass signals in LCT domain. The sampling expansion which is constructed by the ordinary convolution in the time domain can reduce the effect of spectral leakage and is easy to implement. The classical multichannel sampling theorem and the well-known sampling theorems for the LCT are shown to be special cases of it. Some potential applications of the multichannel sampling are also presented to show the advantage of the theory.     

High resolution dynamic cardiac MRI using partial separability of spatiotemporal signals with a novel sampling scheme

The partial separability (PS) of spatiotemporal signals has been exploited to accelerate dynamic cardiac MRI by sampling two datasets (training and imaging datasets) without breath-holding or ECG triggering. According to the theory of partially separable functions, the wider the range of spatial frequency components covered by the training dataset, the more accurate the temporal constraint imposed by the PS model. Therefore, it is necessary to develop a new sampling scheme for the PS model in order to cover a wider range of spatial frequency components. In this paper, we propose the use of radial sampling trajectories for collecting the training dataset and Cartesian sampling trajectories for collecting the imaging dataset. In vivo high resolution cardiac MRI experiments demonstrate that the proposed data sampling scheme can significantly improve the image quality. The image quality using the PS model with the proposed sampling scheme is comparable to that of a commercial method using retrospective cardiac gating and breath-holding. The success of this study demonstrates great potential for high-quality, high resolution dynamic cardiac MRI without ECG gating or breath-holding through use of the PS model and the novel data sampling scheme.     

Stochastic bounded consensus tracking of leader-follower multi-agent systems with measurement noises based on sampled data with general sampling delay

In this paper we provide a unified framework for consensus tracking of leader-follower multi-agent systems with measurement noises based on sampled data with a general sampling delay. First, a stochastic bounded consensus tracking protocol based on sampled data with a general sampling delay is presented by employing the delay decomposition technique. Then, necessary and sufficient conditions are derived for guaranteeing leader-follower multi-agent systems with measurement noises and a time-varying reference state to achieve mean square bounded consensus tracking. The obtained results cover no sampling delay, a small sampling delay and a large sampling delay as three special cases. Last, simulations are provided to demonstrate the effectiveness of the theoretical results.     

Robustness estimation of software-synchronized all-optical sampling for fiber communication systems

The robustness of the software-synchronized all-optical sampling for optical performance monitoring is estimated for 10-Gb/s fiber communication systems. It reveals that the software-synchronized algorithm is sensitive to the signal degradation caused by chromatic dispersion and nonlinearity in optical fibers. The influence of timing jitter and amplitude fluctuation of the sampling pulses is also investigated. It is found that stringent requirements are imposed on the quality of the sampling pulse and the tolerance of 1-dB Q penalty is measured. Considering the practically available optical sampling pulse sources, the results indicate that the amplitude fluctuation of the sampling pulses has the dominant impacts on the software-synchronized method.     

深穿透粒子输运问题的自适应抽样方法

屏蔽计算中的深穿透问题一直是蒙特卡罗计算的一个难题,研究了一种发射点作为驿站的随机游动机制,推导了相应的自适应抽样方法。其主要优势在于,在蒙特卡罗方法求解粒子输运的同时,利用已经获得的信息,自适应地控制各次抽样数,不断完善计算进程。通过对碰撞点引进重要性函数,实现发射点作为驿站的重要性抽样,并结合自适应控制达到最佳抽样状态。数值结果表明:基于发射点作为驿站的自适应抽样方法,在一定程度上克服了深穿透计算中估计值偏低现象。相应的重要函数抽样方法获得了满意的结果。     

亚ns激光脉冲电取样方法

 针对亚ns激光脉冲,提出一种基于ps脉冲传输线和高速电子电路的电快脉冲取样方法,设计了选通脉冲产生电路和高速取样保持电路,给出一种提高实时采样速率的交叉采样方法,该方法由并联结构的多组取样门阵列实现。可应用于多路激光脉冲的精密测量。     

激光喇曼光谱的光纤采样技术

设计了一种高效光纤喇曼探针，与常规采样方法相比，简化了操作，改善了测量的信噪比，同时，光纤采样实现了远距离测量，使恶劣环境下的现场检测成为可能。     

Deterministic schedules for robust and reproducible non-uniform sampling in multidimensional NMR

We show that a simple, general, and easily reproducible method for generating non-uniform sampling (NUS) schedules preserves the benefits of random sampling, including inherently reduced sampling artifacts, while removing the pitfalls associated with choosing an arbitrary seed. Sampling schedules are generated from a discrete cumulative distribution function (CDF) that closely fits the continuous CDF of the desired probability density function. We compare random and deterministic sampling using a Gaussian probability density function applied to 2D HSQC spectra. Data are processed using the previously published method of Spectroscopy by Integration of Frequency and Time domain data (SIFT). NUS spectra from deterministic sampling schedules were found to be at least as good as those from random schedules at the SIFT critical sampling density, and significantly better at half that sampling density. The method can be applied to any probability density function and generalized to greater than two dimensions.     

基于太赫兹光非对称解复用器结构的低开关能量、高线性度全光采样门实验研究

利用多量子阱结构的非线性半导体光放大器(SOA)构建的太赫兹光非对称解复用器(TOAD), 实验实现了一个开关能量低至25 fJ, 线性度高达0.99的全光采样门. 详细分析了采样脉冲功率和非对称偏移量分别对采样窗口形状、宽度和幅度的影响, 并研究了不同采样窗口宽度下TOAD的开关能量及线性度的变化规律.     

Prospects for the LIBS study of films using an axicon optical system

The features of the application of an axicon optical system for the LIBS sampling are described. For the axicon sampling, the crater depth is less than that for a conventional spherical lens. The zone of thermal effect around a sampling point is also reduced. The results obtained indicate that the axicon optical system is promising for the thin-film study.